In numerous areas of technology, films are produced on a wide variety of substrates, the thickness of the films and further parameters being monitored at least in the manner of spot checks in order to regulate the production process in the films or to control subsequent process steps, for example a lateral patterning. The production of a thin semiconductor film on a semiconductor substrate shall be mentioned as an example. The thin semiconductor film is subsequently patterned laterally in order for example to produce a microelectronic or micromechanical component therein. In many cases, thin semiconductor films are also produced such that they are laterally patterned from the outset. In order to be able to measure and monitor the thickness (and other properties) of a thin semiconductor film whose lateral structures have sizes down to a few 10 nm, film thickness analysis areas (FTA sites) are generally provided in semiconductor structures. A film thickness analysis area is typically provided at each chip, so that a wafer, generally comprising a few hundred chips, has a corresponding number of film thickness analysis areas.
In order to measure the film thickness of the semiconductor film, the wafer is firstly oriented roughly on the basis of its groove or notch or cutout and fitted on a table. Afterward, one or more images of the patterned surface of the wafer or of partial regions thereof are recorded and processed by an image or pattern recognition program. The results of the image recognition are subsequently used for a fine adjustment or an exact orientation of the wafer. A measuring instrument then moves to the known sites of the film thickness analysis areas individually. A measured value of the film thickness of the thin semiconductor film is determined at each film thickness analysis area. This is generally done optically, in particular reflectometrically or else ellipsometrically. Once a measured value of the film thickness of the thin semiconductor film has been determined in this way from each film thickness analysis area, the film thickness determined over the wafer, its variation or else systematic variations over the wafer can be calculated in order to characterize the thin semiconductor film on the substrate.
The conventional method described is very time-consuming. In particular, the pattern recognition at the image or images of the wafer surface that is required prior to the fine adjustment of the wafer is computationally intensive and makes a considerable contribution to the fact that the characterization of a film on a wafer takes a long time in the conventional manner. What is particularly disadvantageous in this case is that the wafer has to remain at the measuring device throughout the period of time of the procedure, which may be several minutes long. The throughput of such a conventional measuring device for film thickness analysis is therefore low. Consequently, only a small proportion of all wafers, for example one from each batch, is measured. The conventional method described thus enables only an incomplete monitoring of the film thickness in conjunction with high operating costs (cost of ownership).
A further disadvantage of the conventional method described is that, for each new layout of a wafer, it is necessary to create a new recipe or a new sequential program for the measurement method described. This sequential program has to define, in particular for each new layout, the arrangement of the individual chips, (size, periodicity) on the wafer and the film thickness analysis areas on the individual chip. Furthermore, the image recognition program or the pattern recognition has to be trained to the new layout. In the case of reflectometric or ellipsometric measurements using a film construction model, it is furthermore necessary for the model to be correspondingly changed or adapted each time the film construction is changed. The described changes in the recipe or sequential program in the case of changes to the layout are time-consuming and cost-intensive. They have more of a disadvantageous effect the smaller the number of wafers produced or chips produced. The recipe creation and adaptation and the costs thereof become all the more serious cost factors as the trend leads to production of an ever greater diversity of products within a factory and to ever more rapid renewal or alteration of products and technologies used.
As alternatives to the above-described measuring devices in which, after a fine adjustment of the wafer, in a targeted manner exclusively the film thickness analysis areas are moved to and measured individually, systems already exist in which the entire wafer is scanned or measurements are effected at a multiplicity of measurement points distributed over the entire wafer, from the results of which measurements it is possible to determine the film thickness. So-called scanners scan the entire wafer surface reflectometrically along a regular grid or a periodic lattice.
In another system, the entire wafer surface is detected in a spatially resolved manner by means of a CCD (CCD=charge coupled (optoelectronic) device) and by means of a plurality of wavelength-selective filters optically connected upstream, spectral information also being obtained by means of the filters.
Both systems operate reflectometrically in order to obtain the film thickness from the wavelength dependence of the reflectivity of the surface of the wafer by using a model for the film on the wafer. However, similarly to the measurement method described in the introduction, both systems require knowledge of the size and arrangement of the chips on the wafer and the size and arrangement of the film thickness analysis areas on the chips. These parameters are necessary as input variables in order to select, from the total set of measurement results obtained on the entire wafer, those which have been obtained at film thickness analysis areas. Consequently, these two systems also have to be newly adapted to every new layout of a chip or a wafer.